Plexifilamentary sheets

ABSTRACT

A sheet comprising a plexifilamentary film-fibril network consisting essentially of high density polyethylene in an overlapping multi-directional configuration, the sheet, as-produced from a spin cell, having a normalized Frazier air permeability of between 0.002 and 0.2 m 3 /m 2 ·minute at 50 gsm, a BET surface area of between 9 and 20 m 2 /gm, and a normalized hydrohead of between 150 and 250 cm at 50 gsm, wherein the plexifilamentary film-fibril network in an overlapping multi-directional configuration is flash-spun from a spin fluid comprising polymer and spin agent, the spin fluid temperature being 190° C. or greater, the spin agent comprising dichloromethane, the spin fluid having a polymer concentration of 12 to 14 weight percent.

FIELD OF THE INVENTION

This invention relates to flash-spun plexifilamentary sheets or fabricssuited for home wrap, protective apparel, and other end use applicationsin which a sheet or fabric must demonstrate both good barrier propertiesand a low air permeability.

BACKGROUND OF THE INVENTION

House wrap is used to wrap the exterior surface of a house or otherbuilding during its construction and, more particularly, afterattachment of sheathing and prior to installation of siding/cladding.

House wrap commonly comprises a barrier layer which provides a moisturebarrier against outside water or moisture, yet allows water vaportransmission from the interior of the housing. In this manner, thepassage of liquid water and air (e.g., rain and wind) into the buildingstructure is restricted, thereby preventing water damage to insulationand structural members and minimizing air movement within the walls. Atthe same time, water vapor which enters the walls from the interior ofthe building structure can exit so that it does not condense within thewall and potentially damage insulation and structural members. Typicalhouse wrap barrier materials include spunbonded high densitypolyethylene fibers sold under the trade designation “DuPont™ Tyvek®HomeWrap®” by E.I. Du Pont de Nemours and Company, Wilmington, DE:non-woven barrier material sold under the trade designation “DuPont™Tyvek® CommercialWrap®” by E.I. Du Pont de Nemours and Company,Wilmington, DE; high density, cross-laminated microperforatedpolyethylene sheet material sold under the trade designation“Rufco-wrap” by Raven Industries, Inc., Sioux Falls, SD; and thecross-woven microperforated polyolefin sheet materials sold by AmocoFoam Products Company, Atlanta, GA, and Fabrene Inc., Mississauga,Ontario, Canada under the trade designations “Amowrap Housewrap” and“Air-Gard® Housewrap,” respectively.

House wrap needs a low Frazier air permeability but with still a highhydrohead. Accordingly, a need exists for providing a protective wrapthat improves energy efficiency and protection against air infiltrationand moisture build-up in buildings while satisfying newly implementedindustry-wide energy and building code regulations. There is also a needfor employing a protective wrap which meets or exceeds the newlyimplemented code requirements on existing framing structures or openingsand/or without increasing the wall profile of a building.

SUMMARY OF THE INVENTION

The present invention is directed to a sheet having a normalized Frazierair permeability of between 0.002 and 0.2 m³/m²·minute @ 50 grams persquare meter (gsm), and a normalized hydrohead of between 150 and 250 cm@ 50 gsm.

A further embodiment of the invention has a BET surface area of at least9 m²/g,

In a still further embodiment, the sheet has a normalized Frazier airpermeability of less than or equal to 0.09 m³/m²·minute @ 50 gsm. Thesheet may also have a normalized Frazier air permeability of greaterthan or equal to 0.0075 m³/m²·minute @ 50 gsm.

In a still further embodiment, the sheet has a normalized hydrohead ofbetween 165 and 208 cm @ 50 gsm.

In a still further embodiment, the sheet has a BET surface area ofbetween 9 and 25 m²/g, or even a BET surface area of between 9 and 20m²/g.

The sheet may also have a basis weight of greater than or equal to 30grams per square meter.

The invention is further directed to a multilayer structure comprising amultiplicity of two or more sheets wherein at least one sheet is a sheetcomprising a plexifilamentary structure having a normalized Frazier airpermeability of between 0.002 and 0.2 m³/m²·minute @ 50 gsm,

-   -   I. a normalized hydrohead of between 150 and 250 cm @ 50 gsm,        and    -   II. a basis weight of greater than or equal to 30 gsm.

A further embodiment of the multilayer structure of the invention has aBET surface area of at least 9 m²/g,

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic, not to scale, cross sectional view of a spincell illustrating a process for making flash-spun plexifilamentarysheets.

DETAILED DESCRIPTION

Applicants specifically incorporate the entire contents of all citedreferences in this disclosure. Further, when an amount, concentration,or other value or parameter is given as either a range, preferred range,or a list of upper preferable values and lower preferable values, thisis to be understood as specifically disclosing all ranges formed fromany pair of any upper range limit or preferred value and any lower rangelimit or preferred value, regardless of whether ranges are separatelydisclosed. Where a range of numerical values is recited herein, unlessotherwise stated, the range is intended to include the endpointsthereof, and all integers and fractions within the range. It is notintended that the scope of the invention be limited to the specificvalues recited when defining a range.

The term “polymer” as used herein, generally includes but is not limitedto, homopolymers, copolymers (such as for example, block, graft, randomand alternating copolymers), terpolymers, etc. and blends andmodifications thereof. Furthermore, unless otherwise specificallylimited, the term “polymer” shall include all possible geometricalconfigurations of the material. These configurations include, but arenot limited to isotactic, syndiotactic and random symmetries.

The term “polyethylene” as used herein is intended to encompass not onlyhomopolymers of ethylene, but also copolymers wherein at least 85% ofthe recurring units are ethylene units.

The terms “nonwoven fabric”, “nonwoven sheet” or “nonwoven web” as usedherein mean a structure of individual fibers or threads that arepositioned in a random manner to form a planar material without anidentifiable pattern, such as would be seen in a knitted fabric.

As used herein, the “machine direction” is the long direction within theplane of a sheet, i.e., the direction in which the sheet is produced.The “cross direction” is the direction within the plane of the sheetthat is perpendicular to the machine direction.

The term “plexifilamentary” as used herein, means a three-dimensionalintegral network of a multitude of thin, ribbon-like, film-fibrilelements of random length and a median fibril width of less than about25 microns. In plexifilamentary structures, the film-fibril elements aregenerally coextensively aligned with the longitudinal axis of thestructure and they intermittently unite and separate at irregularintervals in various places throughout the length, width and thicknessof the structure to form a continuous three-dimensional network.

The term “spin fluid” refers to the total composition that is spun usingthe spinning apparatus described herein. Spin fluid includes polymer andspin agent.

The term “spin agent” refers to the processing medium or mixture ofmedia that is used to initially dissolve the polymer to form the spinfluid.

By “consisting essentially of” means herein that the claimed itemcontains a preponderance of a component, but may contain other itemsthat are added to improve or modify the claimed item's functionalperformance. For example an item that consists essentially ofpolyethylene may also contain fillers, antioxidants, and other additivesthat modify its performance or function.

Test Methods

In the description, examples, and claims, the following test methodswere employed to determine various reported characteristics andproperties. ASTM refers to the American Society for Testing andMaterials, and TAPPI refers to the Technical Association of the Pulp andPaper Industry.

The BET surface area of the plexifilamentary film-fibril web product isanother measure of the degree and fineness of fibrillation of theflash-spun product. Surface area is measured by the BET nitrogenabsorption method of S. Brunauer, P. H. Emmett and E. Teller, J. Am.Chem. Soc., V. 60 p 309-319 (1938) and is reported as m²/g. BET surfacearea is measured using a Quantachrome model NOVA 3000e.

Basis Weight was determined by ASTM D-3776, which is hereby incorporatedby reference, and is reported in g/m² or gsm. The basis weights reportedfor the examples below are each based on an average of at least twelvemeasurements made on the sample.

Gurley Hill (or just “Gurley”) Porosity is a measure of the permeabilityof the sheet material for gaseous materials. In particular, it is ameasure of how long it takes a volume of gas to pass through an area ofmaterial wherein a certain pressure gradient exists. Gurley Hillporosity is measured in accordance with TAPPI T-460 OM-88 using aLorentzen & Wettre Model 121D Densometer. This test measures the timerequired for 100 cubic centimeters of air to be pushed through a 28.7 mmdiameter sample (having an area of one square inch) under a pressure ofapproximately 1.21 kPa (4.9 inches) of water. The result is expressed inseconds that are frequently referred to as Gurley Seconds.

Frazier air permeability is a measure of air permeability of porousmaterials and is reported in units of ft³/min per ft². Frazier airpermeability is measured in accordance with ASTM D737-04. It measuresthe volume of air flow through a material at a differential pressure of0.5 inches water column (equivalent to 124.5 Pa). An orifice is mountedin a vacuum system to restrict flow of air through sample to ameasurable amount. The size of the orifice depends on the porosity ofthe material. Frazier air permeability, which is also referred to asFrazier porosity, is measured using a Sherman W. Frazier Co. dualmanometer with calibrated orifice units in ft³/(ft² min), which wereconverted to m³/(m² min) for purposes of reporting here. Frazier isreported here as normalized to 50 grams per square meter (gsm) of basisweight. If only the Gurley Air Porosity was measured on a sample, theFrazier was calculated according to the following equation:Frazier (m³/m² min)×Gurley Air Porosity (seconds)=0.945.

To facilitate comparison of the Frazier of sheet of different basisweight it is convenient to normalize the Frazier to a sheet with a basisweight of 50 gram per square meter. The normalized Frazier for a basisweight of 50 gram per square meter is determined for the followingrelationship:

${{normalized}\mspace{14mu}{Frazier}\mspace{11mu}{\left\lbrack \frac{m^{3}}{\;{m^{2\mspace{14mu}}\min}} \right\rbrack@\; 50}\mspace{14mu}{gsm}} = {{{Frazier}\;\left\lbrack \frac{m^{3}}{m^{2}\mspace{14mu}\min} \right\rbrack} \times \frac{{basis}\mspace{14mu}{{weight}\mspace{14mu}\lbrack{gsm}\rbrack}}{50\mspace{14mu}{gsm}}}$

Hydrostatic Head (or hydrohead) is a measure of the resistance of thesheet to penetration by liquid water under a static load. A 7 inch×7inch (17.78 cm×17.78 cm) sample is mounted in a SDL 18 ShirleyHydrostatic Head Tester (manufactured by Shirley Developments Limited,Stockport, England). Water is pumped against one side of a 102.6 cmsection of the sample which is supported by a 30 mesh scrim with wireshaving a diameter of about 0.28 mm at a rate of 60+/−3 cm/min untilthree areas of the sample are penetrated by the water. The hydrostaticpressure is measured in inches, converted to SI units and given incentimeters of static water head. The test generally follows ASTM D 583(withdrawn from publication November, 1976).

Hydrohead is reported here as normalized to a basis weight of 50 gramper square meter according to the following equation:

${{normalized}\mspace{14mu}{{{hydrohead}\mspace{14mu}\lbrack{cm}\rbrack}@50}\mspace{14mu}{gsm}} = {{hy}\;{{drohead}\mspace{11mu}\lbrack{cm}\rbrack} \times \frac{50\mspace{14mu}{gsm}}{{basis}\mspace{14mu}{{weight}\mspace{14mu}\lbrack{gsm}\rbrack}}}$

Embodiments of the Invention

The present invention is directed to a sheet comprising aplexifilamentary structure, the sheet having a normalized Frazier airpermeability of between 0.002 and 0.2 m³/m²·minute @ 50 gsm, and anormalized hydrohead for a basis weight of 50 gram per square meter ofbetween 150 and 250 cm @ 50 gsm. The terminology “@ 50 gsm” refers tothe normalization procedures given above, where a measurement for anyarbitrary basis weight is normalized to 50 gsm.

In a further embodiment the sheet consists essentially of polyethylene.

In a still further embodiment the BET surface area of the sheet is atleast 9 m²/g

In a further embodiment, the sheet has a normalized Frazier airpermeability of less than or equal to 0.09 m³/(m² minute) @ 50 gsm. Thesheet may also have a normalized Frazier air permeability of greaterthan or equal to 0.0075 m³/m²·minute @ 50 gsm.

In a still further embodiment, the sheet has a normalized hydrohead ofbetween 165 and 208 cm @ 50 gsm.

In a still further embodiment, the sheet has a BET surface area ofbetween 9 and 25 m²/g, or even a BET surface area of between 9 and 20m²/g.

The sheet may also have a basis weight of greater than or equal to 30grams per square meter.

In a further embodiment the flash spun plexifilamentary fiber strand ofany of the embodiments described herein may be consolidated into a sheetstructure. This sheet structure may then be optionally thermally ormechanically bonded.

The invention is further directed to a multilayer structure comprising amultiplicity of two or more sheets wherein at least one sheet is apolyethylene sheet comprising a plexifilamentary structure having

-   -   I. a normalized Frazier air permeability of between 0.002 and        0.2 m³/m²·minute @ 50 gsm,    -   II. a normalized hydrohead of between 150 and 250 cm @ 50 gsm,        and    -   III. a basis weight of greater than or equal to 30 grams per        square meter (gsm).

A further embodiment of the multilayer structure of the inventioncomprises a plexifilamentary sheet having a BET surface area of at least9 m²/g,

The process for making flash-spun plexifilamentary sheets, andspecifically Tyvek® spunbonded olefin sheet material, was firstdescribed in U.S. Pat. No. 3,081,519 to Blades et al. (assigned toDuPont.) The '519 patent describes a process wherein a solution ofpolymer in a liquid spin agent that is not a solvent for the polymerbelow the liquid's normal boiling point, at a temperature above thenormal boiling point of the liquid, and at autogenous pressure orgreater, is spun into a zone of lower temperature and substantiallylower pressure to generate plexifilamentary film-fibril strands. Asdisclosed in U.S. Pat. No. 3,227,794 to Anderson et al. (assigned toDuPont), plexifilamentary film-fibril strands are best obtained usingthe process disclosed in Blades et al. when the pressure of the polymerand spin agent solution is reduced slightly in a letdown chamber justprior to flash-spinning.

The general flash-spinning apparatus chosen for illustration of thepresent invention is similar to that disclosed in U.S. Pat. No.3,860,369 to Brethauer et al., hereby incorporated by reference. Asystem and process for flash-spinning a fiber-forming polymer is fullydescribed in U.S. Pat. No. 3,860,369, and is shown in FIG. 1 . Theflash-spinning process is normally conducted in a chamber 10, sometimesreferred to as a spin cell, which has a spin agent removal port 11 andan opening 12 through which non-woven sheet material produced in theprocess is removed. A spin fluid, comprising a mixture of polymer andspin agent, is provided through a pressurized supply conduit 13 to aspinning orifice 14. The spin fluid passes from supply conduit 13 to achamber 16 through a chamber opening 15. In certain spinningapplications, chamber 16 may act as a pressure letdown chamber wherein areduction in pressure causes phase separation of the spin fluid, as isdisclosed in U.S. Pat. No. 3,227,794 to Anderson et al. A pressuresensor 22 may be provided for monitoring the pressure in the chamber 16.

The spin fluid in chamber 16 next passes through spin orifice 14. It isbelieved that passage of the pressurized polymer and spin agent from thechamber 16 into the spin orifice generates an extensional flow near theapproach of the orifice that helps to orient the polymer. When polymerand spin agent discharge from the orifice, the spin agent rapidlyexpands as a gas and leaves behind fibrillated plexifilamentaryfilm-fibrils. The gas exits the chamber 10 through the port 11.Preferably, the gaseous spin agent is condensed for reuse in the spinfluid.

The polymer strand 20 discharged from the spin orifice 14 isconventionally directed against a rotating deflector baffle 26. Therotating baffle 26 spreads the strand 20 into a more planar structure 24that the baffle alternately directs to the left and right. As the spreadfiber strand descends from the baffle, the fiber strand iselectrostatically charged so as to hold the fiber strand in a spreadopen configuration until the fiber strand 24 reaches a moving belt 32.The fiber strand 24 deposits on the belt 32 to form a sheet 34. The beltis grounded to help ensure proper pinning of the charged fiber strand 24on the belt. The fibrous sheet 34 may be passed under a roller 31 thatcompresses the sheet into a lightly consolidated sheet 35 formed withplexifilamentary film-fibril networks oriented in an overlappingmulti-directional configuration. The sheet 35 exits the spin chamber 10through the outlet 12 before being collected on a sheet collection roll29.

A “thermally consolidated” or “thermally bonded” sheet is a sheet madeby thermal consolidation of a web of the invention. Some examples ofthermal bonding processes are through gas bonding, steam entanglement,ultra-sonic bonding, stretched bonding, hot calendaring, hot rollembossing, hot surface bonding.

Thermal surface bonding can be performed by a process as described inU.S. Pat. No. 3,532,589 to David for hard bonded surfaces. In thisprocess the plexifilamentary sheet passes subsequently over a heateddrum-cooling drum-heating drum-cooling drum to thermally bond both sidesof the material. The heating drum is kept at a temperature that wouldresult in partial melting of the plexifilamentary structure to includethe bonding of the sheet. The cooling drum has the purpose to reduce thetemperature to a value where the sheet will not shrink nor distort whenunrestrained. During the bonding process the sheet is slightlycompressed by a flexible belt to have a controlled shrinkage.

Alternatively, the plexifilamentary sheet may be bonded by means ofembossing rolls and rubber coated back-up roll to bond one or two sidesof the sheet. The embossing roll can be smooth or contain differentpatterns, for example, but not limited to those shown in the followingreferences, namely a point pattern (U.S. Pat. Nos. 3,478,141, 6,610,390US 2004/241399 A1), a rib pattern (US2003/0032355 A1), a random pattern(U.S. Pat. No. 7,744,989) or different patterns (U.S. Pat. No.5,964,742). The sheet may pass through one or multiple stations of anembossing roll with rubber coated back-up roll. In addition, before andafter the pairs of embossing and back-up rolls the sheet may be incontact with pre-heat or cooling rolls as described in U.S. Pat. No.5,972,147. Finally, the bonding process the material may be softened,for example, a button breaking device as described in U.S. Pat. No.3,427,376 by Dempsey.

Examples

Spin fluids were prepared, flash-spun and formed into a consolidatedsheet according to the process described in U.S. Pat. No. 3,860,369 anddescribed above. The polymer concentrations reported in the exampleswere calculated as the weight percent of polymer based on the total spinfluid weight, where the total spin fluid weight includes the weight ofpolymer and of the spin agent.

Unless otherwise indicated, the plexifilamentary webs and sheetsprepared in the present examples were flash-spun using a spin agentconsisting of 81 weight percent dichloromethane and 19 weight percent2,3-dihydrodecafluoropentane. Comparative examples were made with a spinagent comprising n-pentane. The sheet of the invention resulted from aflash spinning process conducted from an upstream pressure letdownchamber of at least 15 cm³ and a discharge pressure of 75 bar gaugeminimum, yielding a fiber of 300 to 400 denier.

The polymer used in all of the examples was high density polyethylenehaving a melt index of 2.35 g/10 min (measured according to EN ISO 1133at 190° C. and 5 kg load), and 24.5 g/10 min (measured according to ENISO 1133 at 190° C. and 21.6 kg load) a density of 0.96 g/cm³ (measuredaccording to EN ISO 1183). The polymer used in the comparative examplewas high density polyethylene having a melt index of 0.96 g/10 min(measured according to ASTM D 1238 at 190° C. and 2.16 kg load) and 34.4g/10 min (measured according to ASTM D 1238 at 190° C. and 21.6 kgload).

Table 1 summarizes spinning conditions for the examples and comparativeexamples.

TABLE 1 Polymer Concentration Spin Fluid Weight Percent of Spin PressureTemperature Sample Spin Fluid (barg) (° C.) Example 1 12 85.5 195Example 2 14 76.0 190 Example 3 13 83.0 190 Comparative 17 76.0 192Example 1 Comparative 17 81.1 194 Example 2 Comparative 17 85.6 198Example 3 Comparative 17 90.6 200 Example 4 Comparative 17 90.9 201Example 5 Comparative 21.3 40.0 180 Example 7 Comparative 17.1 67.9 190Example 8 Comparative 17.9 59.7 185 Example 9

Table 2 summarizes the properties of the flash spun plexifilamentarysheets that were obtained. Comparative examples 7 to 9 describe thenormalized Frazier air permeability and normalized hydrohead @50 gsm foradditional basis weight values. The BET surface area was not measured(n.m.).

TABLE 2 Normalized Frazier Air Normalized Basis Permeability HydroheadBET Surface Weight (m³/m² · minute (cm @ Area Sample (gsm) @ 50 gsm) 50gsm) (m²/g) Example 1 41.4 0.089 165 9.8 Example 2 42.3 0.0075 174 18.5Example 3 41.0 0.016 208 13.0 Comparative 61.0 0.013 104 21.6 Example 1Comparative 60.0 0.018 105 19.1 Example 2 Comparative 61.7 0.04 95 15.4Example 3 Comparative 60.7 0.065 100 15.4 Example 4 Comparative 59.70.065 101 13.7 Example 5 Comparative 66.5 0.0024 69.3 n.m. Example 7Comparative 40.7 0.076 106.8 n.m. Example 8 Comparative 54.2 0.015 77.8n.m. Example 9

None of the comparative examples achieve the hydrohead performance ofthe examples of the invention, which exhibit a unique combination ofperformance in terms of the selected comparison criteria of Table 2.

We claim:
 1. A sheet comprising a plexifilamentary film-fibril networkconsisting essentially of high density polyethylene in an overlappingmulti-directional configuration, the sheet, as-produced from a spincell, having a normalized Frazier air permeability of between 0.002 and0.2 m³/m²·minute at 50 gsm, a BET surface area of between 9 and 20m²/gm, and a normalized hydrohead of between 150 and 250 cm at 50 gsm,wherein the plexifilamentary film-fibril network in an overlappingmulti-directional configuration is flash-spun from a spin fluidcomprising polymer and spin agent, the spin fluid temperature being 190to 195° C., the spin agent comprising a mixture of processing mediacomprising dichloromethane, the spin fluid having a polymerconcentration of 12 to 14 weight percent.
 2. The sheet of claim 1wherein the sheet as-produced from the spin cell has a normalizedFrazier air permeability of less than or equal to 0.09 m³/m²·minute at50 gsm.
 3. The sheet of claim 1 wherein the sheet as-produced from thespin cell has a Frazier air permeability of greater than or equal to0.0075 m³/m²·minute at 50 gsm.
 4. The sheet of claim 1 wherein the sheetas-produced from the spin cell has a normalized hydrohead of between 165and 208 centimeters at 50 gsm.
 5. The sheet of claim 1 wherein the sheetas-produced from the spin cell has a basis weight of greater than orequal to 30 grams per square meter.
 6. A multilayer structure comprisinga multiplicity of two or more sheets wherein at least one sheet is asheet according to claim
 1. 7. A thermally consolidated sheet made fromsheet according to claim
 1. 8. A multilayer structure comprising amultiplicity of two or more sheets wherein at least one sheet is a sheetaccording to claim 7.